High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGT) allow the intake airflow to be optimized over a range of engine speeds. This is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
In compression ignition engines equipped with both VGT and EGR systems, optimal steady-state performance in terms of fuel economy and emissions is achieved by coordinating the operation of the two actuators. Typical engine designs utilize a compressor mass airflow (MAF) sensor and a manifold absolute pressure (MAP) sensor for proper regulation of airflow into the engine and, consequently, EGR flow in VGT-equipped engines. Regulation of airflow is important because it directly relates to the amount of fuel that can be injected to meet driver demand. For a given engine speed and requested fueling rate, the control algorithm looks up the desired values for MAP and MAF and controls the EGR to achieve the desired MAF and the VGT to achieve the desired MAP.
The sensor set selection, thus, determines the control system cost and performance. For example, MAF sensors have limited accuracy and are more expensive than MAP sensors or exhaust manifold pressure (EXMP) sensors. The difficulty in calibrating the engine control strategy is also directly related to the sensors used to control the engine. Consequently, there is a need for an engine control system having a low-cost sensor set, which is easy to calibrate, and provides robust engine performance.